This invention relates to the field of high-density electronic circuits which are embedded in insulating materials such as ceramics. The invention has applications in any field in which it is necessary to provide closely-spaced conductors in a three-dimensional insulating substrate.
One area in which the present invention is useful is in the manufacture of plasma displays. In a plasma display, front and back panels define a chamber which contains a gas. The front panel is clear, and the back panel has a plurality of closely-spaced conductors, arranged in a matrix. By applying suitable voltages to selected conductors, certain regions of the gas are ionized to become a plasma, and are induced to generate light. An optical filter placed near the front panel determines the color of the light as seen by an outside observer.
The preferred structure for the back panel is an insulating substrate that has conductors arranged in grooves formed in the substrate. Because the voltages needed to generate the plasma are high, it is important to isolate each conductor from its neighbors, to avoid the risk of arcing.
The method known in the prior art, for making insulating substrates having grooves containing conductive material, is to screen print the conductive material onto a flat surface of unfired ceramic material, and then to press the conductive material into the unfired ceramic. The pressure forms grooves in the ceramic, and the conductive material is forced into these grooves.
A major problem with the technique described above is that it cannot be practiced with precision when the density of conductors is high. It is very difficult to achieve precise alignment between the screened pattern of lines of conductive material, and the embossing die used to embed the conductive material in the ceramic. Typically, under the best circumstances, steel wire screens can be accurately registered and sized to no better than plus or minus 0.001 inches (25 microns) per ten inches of span. For a 20-inch workpiece, the latter tolerance means that there may be an error of at least plus or minus 0.002 inches (50 microns). Since the pitch of a typical pattern of conductors is 165 microns, and the grooves are typically 110 microns wide at their bottoms, the screen mask itself takes up all of the allowable tolerance. The latter does not allow for any process variations in the embossed parts.
The process described above is also likely to result in an xe2x80x9coverflowxe2x80x9d of conductive material within the grooves formed in the ceramic substrate. The conductive material, when pressed into the unfired ceramic, tends to diffuse the conductive material and cause it to flow up over the side wall of the groove, instead of being laid down at the bottom of the groove. The latter effect causes a problem, especially where high voltages are to be applied to the conductors, because electric current is more likely to arc from one conductor to another when the conductors fill the grooves, or when they spill out beyond the grooves. If too much conductive material is present, it is even possible to have short circuits between conductors. Thus, it is desirable, and often necessary, for the conductive material to occupy only the bottom regions of the grooves, to insure maximum electrical isolation between adjacent conductors.
Apart from the problem of possible short circuits, the process of the prior art tends to produce conductors having non-uniform thicknesses, and thus the resistivity of the circuit is increased. The longer the circuit paths and the more closely-spaced the conductors, the more severe the resistivity problem becomes.
In the above-described procedures, the material embedded in the grooves is preferably a conductive cermet material. A cermet is a material containing particles of metal dispersed in a ceramic carrier. Initially, the cermet may take the form of a paste. When fired at high temperatures, the metal particles melt and fuse together, so that the metal particles become an integral electrical conductor which remains conductive when cooled. In this specification, the term xe2x80x9cconductive cermetxe2x80x9d will be used to identify the cermet paste, although it should be understood that the material does not actually become conductive until it has been fired. Examples of such cermet materials are given in U.S. Pat. No. 4,897,676, the disclosure of which is incorporated by reference herein.
The present invention provides a practical and precise method of forming closely-spaced conductors embedded in an insulating substrate. The present invention makes it possible to embed conductive cermet in very closely-spaced grooves, without wasting appreciable cermet. The product made by this process is of such higher quality than comparable circuits of the prior art, that it is possible to use less expensive materials as conductors, without any reduction in the level of performance.
In one aspect, the present invention comprises a method of making a three-dimensional high-density circuit. In this method, one first provides a master tool having walls and grooves which define a desired circuit pattern. Next, one creates an electroform, by electroplating over the master tool, the electroform having a plurality of tongues and grooves which correspond to the grooves and walls of the master tool. Then, one presses the electroform into an unfired ceramic substrate, to form a pattern of grooves and walls in the substrate.
Next, one places a stencil over the substrate, the stencil having a plurality of U-shaped members which mate neatly with the walls formed in the substrate. It is an important feature of the invention that the stencil is made from the same master tool as the electroform, or from a replication thereof, so that the pattern defined by the U-shaped members will be the same as the pattern of walls and grooves in the substrate.
One then applies a cermet paste over the stencil, forcing the paste through spaces between adjacent U-shaped members, until the paste fills the space at the bottom of the grooves of the substrate. The U-shaped members are sized such that their side walls do not extend to the bottom of the grooves of the substrate, so the paste can fill the space near the bottom.
The stencil, with cermet paste remaining between the U-shaped members, is then removed from the substrate, leaving the paste at the bottom of each of the grooves. The substrate can then be fired in a conventional manner.
Another aspect of the invention is the method of making the stencil described above. The stencil is made by electroforming material over the master tool. Before the electroforming begins, one coats the master tool with a photoresist. The master tool is exposed to collimated light, which exposes only the photoresist on the tops and walls of the grooves in the tool, but leaves the photoresist at the bottom of the grooves unexposed. After chemical development, the exposed photoresist is dissolved away, but the photoresist at the bottom of the grooves remains. Since the photoresist is non-conductive, no electroplating occurs at the bottom of the grooves. Thus, the electroforming process results in material that surrounds the walls of the master tool, but does not extend to the bottom of the walls. In short, the material formed has the shape of a plurality of U-shaped members, the U-shaped members having side walls which are not as long as the depth of the grooves in the final product.
In the preferred embodiment, in the manufacture of the stencil, before the electroforming takes place, one places a plurality of stainless steel wires across the walls of the master tool. The wires becomes embedded in the electroformed material, and give it rigidity. Also, the electroforming process produces bridge sections, integral with the U-shaped members, which hold the U-shaped members together, making the stencil a one-piece object.
After the electroforming is complete, the tops of the U-shaped members are lapped down to remove excess electroformed material. The result is a stencil which mates perfectly with a grooved substrate formed from the same master tool.
Another aspect of the present invention is the high-density circuit made by the method described above. This circuit is a ceramic substrate having walls and grooves, and in which a conductive material is located at the bottom of at least some of the grooves. The grooves define circuit lines having a density which can be about 10 to about 200 lines per inch. The height of the conductive material in the grooves is preferably in the range of 0.1-0.6 times the height of the wall. Thus, the walls of the grooves are substantially higher than the height of the conductors. This arrangement enhances the isolation of the conductors from each other.
The present invention therefore has the primary object of providing a high-density electronic circuit.
The invention has the further object of providing a method for making a high-density electronic circuit.
The invention has the further object of reducing the cost of making a high-density electronic circuit.
The invention has the further object of improving the accuracy with which one can make a high-density electronic circuit.
The invention has the further object of providing a high-density electronic circuit in which the conductors have uniform thickness, and reduced resistivity.
The invention has the further object of providing a high-density electronic circuit, which circuit can be used at high voltages without arcing.
The reader skilled in the art will recognize other objects and advantages of the present invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims.